The interaction between motor neurons and muscle cells determines muscle tone and effects muscular contraction, and thereby underlies all voluntary and involuntary movement.
Motor neurons are traditionally classified as upper motor neurons or lower motor neurons. Upper motor neurons reside in the precentral gyrus of the brain, and send long processes down to synapse on lower motor neurons in the ventral (anterior) horns of the grey matter of the spinal cord. From the ventral horns of the spinal cord, axon processes of lower motor neurons coalesce to form the ventral roots. These axons eventually terminate on one or more muscle fibers. Through arborization of the terminal part of its fiber, each lower motor neuron comes in contact with anywhere from a few to 100-200 or more muscle fibers to form a "motor unit," (Adams and Victor, 1985, in "Principles of Neurology," McGraw-Hill, Inc., New York, p. 37).
Motor neuron disorders result in varying degrees of muscle weakness, causing disability which ranges from slight difficulty performing difficult tasks to total paralysis. Lower motor neuron disorders are generally associated with a flaccid paralysis and decrease in muscle tone. Contiguous groups of muscles, innervated by single nerves or whose motor neurons lie close together in the spinal cord, may be affected and atrophy may be quite profound, up to 70-80 percent of total bulk. A diseased motor neuron may become irritable and muscle fibers that it controls may discharge sporadically, in isolation from other units, to produce a visible twitch or fasciculation. In contrast, when upper motor neurons are damaged, a spastic paralysis with increase in muscle tone and hyperactive tendon reflexes generally results. Usually, an entire limb or half of the body, rather than individual muscle groups, is affected. Atrophy is slight and typically results from lack of use. Fasciculations are absent. The identification of a clinical syndrome as representing upper or lower motor neuron damage may facilitate its diagnosis and management.
A wide array of neurological disorders may affect motor neurons. Upper motor neurons, for example, are predominantly affected by cerebrovascular accidents, neoplasms, infections and trauma. Lower motor neurons, or anterior horn cells, are secondarily affected by these processes, but in addition are subject to a number of disorders in which anterior horn cell loss is the primary feature, including amyotrophic lateral sclerosis, infantile and juvenile spinal muscular atrophy, poliomyelitis and the post-polio syndrome, hereditary motor and sensory neuropathies, and toxic motor neuropathies (e.g. vincristine intoxication).
The vinca alkaloid vincristine sulfate is a widely used cancer chemotherapeutic agent that commonly produces a mixed sensorimotor polyneuropathy. Electrophysiological studies performed on patients treated with vincristine showed that the drug caused slowing of motor conduction and impairment of sensory conduction in peripheral nerves (McLeod and Penny, 1969). Vincristine is often the drug of choice for its anti-neoplastic properties because, unlike many anticancer drugs, it is neither emetic nor myelotoxic. However, its use is limited by neurotoxicity for which no specific antidotes have been established. Currently, the only known treatment for neurotoxicity associated with vincristine use has been the discontinuation or reduction of the dose or frequency of administration, or both, of this agent. There have been attempts to decrease vincristine toxicity by treatment with thiamine (Kaplan and Wiernik, 1982), B12 (Kaplan and Wiernik, 1982), folinic acid (Jackson et al., 1983), pyridoxine (Jackson et al., 1984), glutamic acid (Jackson et al., 1988), and ganglioside (Favaro et al., 1988), but motor neuropathy has not been addressed specifically.
Ciliary neurotrophic factor (CNTF) has been observed to promote the survival of embryonic motor neurons (Arakawa et al., 1990, J. Neurosci. 10:3507-3515; Oppenheim et al., 1991, Science 251:1616-1618; Wong et al., 1991, Neurology 41(Supplement 1):696P), prevent the degeneration of facial nerve motor neurons after axotomy (Sendtner et al., 1990, Nature 345:440-441), and allay the progression of motor neuron disease in wobbler (Mitsumoto et al., 1994, Science 265: 1107-1110) and PMN (Sendtner et al., 1992, Nature 358:502) strains of mice. CNTF is one of a number of proteins which exert trophic effects on certain components of the nervous system. The name, CNTF, derives from the first noted activity of this protein: its ability to support the survival of dissociated chick ciliary ganglion cells in culture (Manthorpe and Varon, 1985, in "Growth and Maturation Factors," vol. 3, Guroff ed., John Wiley and Sons, New York, pp. 77-117). Human CNTF, a 200 amino acid residue protein, has been cloned and characterized (Masiakowski et al., International Publication No. WO 91/04316, Int. Appln. No. PCT/US90/05241, published Apr. 4, 1991; Collins et al., U.S. Pat. No. 5,011,914, issued Apr. 30, 1991; Masiakowski et al., 1991, J. Neurochem. 57:1003-1011).
As shown in PCT Publication No. WO 91/04316, published Apr. 4, 1991 and incorporated by reference herein, ciliary neurotrophic factor (CNTF) is capable of promoting the survival of motor neurons in vitro and in vivo. Gelfoam implants containing CNTF facilitated the survival of motor neurons in severed facial nerves of newborn rats. Although it is a neurotrophic factor, CNTF exhibits biological activities that are very different from those exhibited by the neurotrophin family of factors, including brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and nerve growth factor (NGF).
By comparison, hepatocyte growth factor (HGF), also referred to as hepatopoietin A, was originally identified in the serum of partially hepatectomized rats as a potent mitogen for cultured rat hepatocytes (Nakamura et al., 1984; Michalopoulos et al., 1984). HGF has subsequently been purified from rat platelets (Nakamura et al., 1986), human plasma (Gohda et al., 1988; Zarnegar and Michalopoulos, 1989), and rat liver (Asami et al., 1991), and its amino acid sequence has been deduced by cDNA cloning (Nakamura et al., 1989; Miyazawa et al., 1989; Tashiro et al., 1990; Seki et al., 1990). Several reports have revealed closed sequence homology between HGF and scatter factor (SF) (Gherardi and Stoker, 1990; Weidner et al., 1990; Rosen et al., 1990; Coffer et al., 1991), a polypeptide that stimulates dissociation of epithelial cell colonies in monolayer culture (Stoker et al., 1987; Gherardi et al., 1989). Evidence indicating that the 2 factors have identical structure and biological activities has also been presented (Weidner et al., 1991; Naldini et al., 1991; Furlong et al., 1991). Until recently, HGF was considered to have a narrow target cell specificity and to act primarily as a humoral mediator of liver regeneration after partial hepatectomy or hepatic injury.
However, it is possible that HGF is a multifunctional polypeptide that may act in a wide variety of cells, including microglia (DiRenzo et al., 1993) and skeletal muscles (Jennische et al., 1993). Furthermore, it has been shown (Stern and Ireland, 1993) that HGF can cause cultured chick ectodermal cells to become neural, raising the possibility that HGF could be a neural inducing signal during the early development of vertebrate embryos. These observations indicate a possible role for HGF in the developing and/or injured nervous system.
HGF receptor has been recently identified as the c-met protooncogene product (Bottaro et al., 1991; Naldini et al., 1991), a transmembrane protein of 190 kD (p190.sup.MET), composed of 2 disulfide-linked chains: an extracellular 50 kD alpha subunit (p50.alpha.) and a transmembrane 145 kD beta subunit (p145.beta.) endowed with tyrosine kinase activity (Gonzatti-Haces et al., 1988). It is expressed in a wide variety of tissues, most predominantly in tissues of epithelial origin (Chan et al., 1988; Iyer et al., 1990). Recent reports have shown that met is also localized in many cells of neuronal origin, including spinal cord (Sonnenberg et al., 1993), hippocampus, cerebellum, forebrain, cortex, and choroid plexus (Schirmacher et al., 1993) of the developing rat brain, indicating possible roles for HGF on a wide spectrum of cells in the central nervous system.